Humans only use 10% of their brain.

In 1936, Dale Carnegie published "How to Win Friends and Influence People," a self-help manual that became one of the bestselling nonfiction books in American publishing history. The preface, written by journalist Lowell Thomas, included a passage attributed to William James, the founding figure of American psychology: "Professor James said that the average man develops only ten percent of his latent mental ability." Thomas was paraphrasing a concept James had explored, that most people operate below their maximum mental potential, and attaching a specific number to it. The percentage, as far as anyone has since determined, was Thomas's invention. James had written no such figure.

By the 1940s, the ten percent claim was appearing in newspaper columns and self-help tracts without the attribution. By the 1960s, it was being used in teacher training materials and educational psychology texts. By the 1970s and 1980s, advertisements for memory improvement courses and brain training products were citing it as established fact, sometimes attributed to James, sometimes to Albert Einstein, though no documentation connects Einstein to the claim. The myth was a free-floating cultural property, requiring no authority because it had, through repetition, become its own authority.

The neurological claim the myth rests on is false in every sense that neuroscience can examine. Modern brain imaging shows that no region of the intact brain sits permanently inactive. Electroencephalography, functional MRI, and positron emission tomography all demonstrate that every identified brain region shows measurable activity during normal waking life. Different regions are more or less active depending on what a person is doing, motor cortex fires more during movement, visual cortex more during visual tasks, but there is no ten percent that is active while ninety percent lies dormant. Even during sleep, brain activity is continuous and complex.

The confusion may partly trace to the neuroscience of cell populations. Roughly 90 percent of brain cells by volume are glial cells, not neurons. Glial cells do not generate action potentials and do not participate in information transmission the way neurons do. An early misreading of this finding, conflating "cells that don't fire" with "unused brain", may have given the myth a veneer of scientific specificity. But glial cells are not dormant: they regulate neurotransmitter levels, provide metabolic support to neurons, maintain the myelin sheaths that allow signals to propagate efficiently, and perform essential housekeeping functions that neurons cannot perform themselves.

The myth also ignores the evolutionary logic of brain tissue. The human brain consumes approximately 20 percent of the body's resting energy budget, far more than would be metabolically defensible if nine-tenths of it were doing nothing. Brain tissue is among the most metabolically expensive tissue in the body. Evolution is generally not generous with energetically costly structures that serve no function; significant damage to any region of the brain, from stroke to tumor to surgical resection, reliably produces identifiable deficits. There is no region of the brain whose removal leaves function entirely intact, as would be expected if it were part of an unused ninety percent.

Neuroscientists were pointing this out in print by the late 1980s. Barry Beyerstein, a cognitive neuroscientist at Simon Fraser University, published a detailed dissection of the ten percent myth in 1999, tracing its origins, examining its neurological claims, and finding no evidence for any of them. The myth appeared in his analysis to be a product not of any single misreading but of a self-reinforcing cultural narrative: people wanted to believe they were not operating at capacity, because the alternative, that this is what human intelligence looks like running flat out, was less appealing.

The ten percent figure continues to circulate in advertising, motivational speaking, and film. The 2014 movie Lucy, starring Scarlett Johansson, used it as its central premise. Neuroscience textbooks have spent decades explaining why it is false. The explanations have generally circulated more narrowly than the myth.